Tungsten is employed for various purposes in manufacturing semiconductor integrated circuits. H. Kondo et al, Japanese patent publication Kokai No. 52-149990, "Method of Manufacturing Multilayer Wiring", discloses an electrical interconnection structure in which a thin film of a conductor such as tungsten is employed as an etch barrier for patterning aluminum and thereafter serves (in part) as an intermediate interconnection element. In making this structure, a first patterned conductive layer of aluminum is defined over a first insulating layer of silicon dioxide along a surface of a silicon semiconductor substrate. A second insulating layer typically consisting of silicon dioxide is formed over the first aluminum layer and over the exposed adjacent material of the first insulating layer. An aperture, commonly referred to as a "via", is then created through the second insulating layer to expose a surface portion of the first Al layer.
The thin tungsten film is deposited on the entire exposed upper surface of the structure--i.e., on the Al surface portion and on the remainder of the second insulating layer. A second aluminum layer is deposited on the tungsten film and then patterned with an etchant that does not significantly attack tungsten. As a result, the W film acts as an etch stop to prevent any portion of the first Al layer from being etched away. The exposed portions of the W film are now removed with an etchant that does not significantly attack aluminum. In the final structure, the remainder of the W film underlies all of the second patterned Al layer. The tungsten lying between the Al layers then serves as an intermediate connection in the structure.
A disadvantage of Kondo et al is that the tungsten lying between insulating material and the second patterned Al layer can cause single-crystal Al whiskers to emerge from it during operation. This leads to electrical shorts between initially insulated conductive lines, resulting in device failure. In addition, the tungsten on the insulating material increases the height (i.e., thickness) of the composite W/Al layer. The increased height produces a more formidable step profile to cover when one or more further layers are deposited on the structure.
Tungsten is conventionally deposited as a relatively uniform film on a composite surface consisting of different materials as shown, for example, in Kondo et al. However, tungsten can also be selectively deposited on such a composite surface under certain conditions as disclosed in J. J. Cuomo et al, British Pat. No. 1,330,720. In Cuomo et al, a patterned nucleating layer consisting, for example, of aluminum is first formed on a glass substrate. A tungsten layer is then formed on the patterned nucleating layer by chemical vapor deposition (CVD) at or near atmospheric pressure. The tungsten is provided by hydrogen reduction of tungsten hexafluoride in the immediate vicinity of the nucleating layer which serves as a surface for the tungsten to accumulate. The surface portions of the substrate laterally adjacent to the nucleating layer are simultaneously ablated (i.e., removed) during the tungsten CVD. As a result, tungsten only accumulates on the nucleating layer which prevents the underlying substrate portion from being ablated away. Typically, the thickness of the substrate portions ablated away is about the same as the thickness of the deposited W layer.
The selective tungsten deposition capability of Cuomo et al is a significant enhancement because selective deposition is simpler than uniform deposition followed by selective etching. However, the substantial substrate ablation in Cuomo et al is undesirable in many semiconductor applications, particularly in fabricating electrical interconnections of the type in Kondo et al as various structures having multiple thin layers whose thicknesses must be preserved.